The central hypothesis is that activation of Toll-like receptor (TLR) 2 contributes to the connected processes of endothelial dysfunction, coagulopathy, and increased vascular permeability in sepsis. TLR2 mediates the inflammatory effects of bacterial lipoproteins. The studies will define mechanisms by which TLR2 agonists modulate coagulation pathways in endothelial cells, and will assess the functional significance of TLR2 activation on endothelial permeability and coagulopathy in sepsis. These studies will provide insights into the mechanisms of coagulopathy, vascular leak, and respiratory dysfunction in sepsis. TLR2 agonists are present in all of the major classes of microorganisms that cause sepsis. Thus if TLR2 is important in sepsis-induced coagulopathy, vascular leak or respiratory failure, then TLR2 signaling pathways could be suitable targets for sepsis therapies. Specific Aim #1: Define mechanisms by which TLR2 activation modulates endothelial cell (EC) expression of coagulation pathway factors in vitro. Studies will test the hypotheses that TLR2 agonists alter expression of factors involved in coagulation, anticoagulation, and fibrinolysis: 1) through NF-[unreadable]B, and 2) through additional mediators, including TGF-[unreadable], TNF[unreadable], and/or NO. EC will be treated with TLR2 agonists, and expression of tissue factor (TF), tissue factor pathway inhibitor (TFPI), and plasminogen activator inhibitor type 1 (PAI-1) will be quantified. Mechanisms by which TLR2 agonists modulate coagulation pathways will be defined using EC from knockout mice, and using targeted inhibitors with human endothelial cells. Specific Aim #2: Assess the effects of TLR2 activation on endothelial permeability in vitro. Studies will test the hypotheses that TLR2 activation increases endothelial leakiness, as assessed by permeability of EC monolayers to albumin. Specific Aim #3: Define the functional significance of TLR2 activation on coagulopathy and on lung vascular permeability in sepsis. Sepsis will be induced in mice using peritonitis and pneumonia models. Studies will compare responses of TLR2 knockout mice with those of wild-type mice, and will assess the relative importance of TLR2 in the pathophysiology of Gram- positive versus Gram-negative sepsis. Blood coagulation times will be measured, and levels of factors involved in coagulation and fibrinolysis will be quantified in blood and in lung. Lung vascular leakiness will be assessed using lung wet:dry weight ratios and permeability to albumin. Histologic analyses will assess for microvascular thrombosis, architectural changes, evidence of pulmonary edema, and lung expression of PAI-1 and TF.